The invention is directed towards the field of printers and more specifically to the field of electrostatic printers.
Electrophotographic (EP) primers generically called laser printers, are becoming increasingly common. Although electrophotography produces high print quality, the process is relatively complex and requires a bulky printing apparatus. An alternative to EP printing is toner ejection printing (TEP), described in U.S. Pat. No. 3,689,935 to Pressman, et al. The print quality of the TEP process theoretically should approach that of EP printers. However, the TEP process uses only two steps rather than the six steps required by conventional EP processes. This consolidation has attracted increasing interest due to the possibility of reduced costs.
FIG. 1A shows a cross-sectional partial schematic view of a conventional toner ejection printer 100 such as is described in Pressman, et al. FIG. 1B shows a view of the printhead 106 shown in FIG. 1A along lines A--A. The printhead 106 of the TEP printer 100 has a plurality of apertures 108 that allow charged toner particles 110 to pass from the toner supply 112 to the back electrode 114. A continuous shield electrode 118 is formed on the surface of the printhead 106 facing the toner supply 112 is coupled to ground. Gate electrodes 120 are formed on the surface of the printhead opposite to the shield electrode 118 facing the back electrode. Individual apertures are selectively opened or closed by applying the appropriate voltage to the corresponding gate electrode 120.
In an alternative TEP configuration, the shield electrode 118 is eliminated (no ground plane) and a single gate electrode layer for addressing individual aperture is used. Although the ground-free configuration has the advantage that a much smaller gate voltage can be used to open and close individual apertures, cross talk can be very significant. Crosstalk is problematic since the charge of neighboring electrodes can affect spot development.
As pixel density increases so does the related interconnect circuitry and driver circuitry necessary to support the added pixel density. In order to drive a large array of pixels, address multiplexing is sometimes used. U.S. Pat. No. 5,353,050 to Kagayama describes a printer where the address electrodes are multiplexed. FIG. 2A shows a cross-sectional partial schematic view of a conventional toner ejection printer 200 where the address electrodes are multiplexed. FIG. 2B shows a top view of the printhead shown in FIG. 2A along lines A--A.
Although multiplexing allows increased pixel density, it has the disadvantage of potentially providing an inconsistent environment for the pixels currently being developed. Consider a first case where an end gate electrode row 204 and an interior gate electrode row 206 are both on simultaneously while the remaining gate electrode rows are off. Assuming a negative toner, a typical potential for the electrode rows that are on would be 0 volts while a typical potential for the electrode rows that are off would be -300 volts. In a second case, the gate electrode rows are sequentially addressed so that electrode rows 204 and 206 are on at a different times. The apertures in electrode row 204 experience a different local environment than the apertures in electrode row 206, since electrode row 204 has only one adjacent row off while electrode row 206 has two adjacent rows off.
Electrostatically charged toner particles react to the electric field. Thus, the difference in the local environment can lead to differences between the amount of toner particles deposited through the aperture that are on in row 204 versus the amount of charge deposited through the aperture that are on in row 206. If the toner particles sense a high electric field, more toner particles can overcome their adhesion to the developer roll causing the toner particle to move more quickly from the developer roll towards the aperture. If the toner particle senses a low electric field, less toner particles overcome their adhesion to the developer roll and toner particles move more slowly towards the aperture. This results in a smaller amount of toner deposited and thus a lighter pixel compared to the pixel in the high electric field case. Although the speed of the toner particle may not be critical in EP printers, toner particle speed is critical in TEP printers because of the small development time window for each pixel.
A toner ejection printer which improves print quality by providing a more uniform local charge environment to improve pixel tonal uniformity is needed.